1. Technical Field of the Invention
This application is based on applications Nos. 8-346010, 8-346013, 8-346014, 8-346015, 8-346016, 8-346017, and 8-346019 filed in Japan, the contents of which are hereby incorporated by reference.
The present invention relates to a sheet feeder unit which supplies cut sheets set in a pile thereon by a feeding roller which contacts to the cut sheets from one side thereof, and particularly to a sheet feeder unit which is capable of separating cut sheets to be supplied one after another by a frictional force generated when a leading end of the cut sheet is pushed forward to contact with a friction member which is supported at one end thereof and extended toward a conveying path of the cut sheet. The present invention may be employed for feeding a plurality of piled cut sheets one by one in various appliances such as a copying machine, printer, facsimile machine, or image reading apparatus.
2. Description of Related Art
Japanese published unexamined patent application No. 7-196186 discloses the like of such sheet feeder unit, which is shown in FIG. 1. The feeder unit according to this prior art has a friction member 933 rotatably supported at its lower end by a shaft 932 to be able to stand upright and tilt, and configured to be rectangular when viewed from the front. Each time a feeding roller 931 is rotated, an uppermost cut sheet 911 is pushed forward so that a whole width of a leading end of the cut sheet 911 comes into frictional contact with the friction member 933 and causes the same to back away against a rebounding force of a spring 934 as shown by phantom lines in FIG. 1. The cut sheet 911 is thereby separated from the other cut sheets, which are prevented from being supplied one upon another.
In order to assure the separation of cut sheets from each other, the angle .alpha. made by a surface of the friction member 933 and an uppermost cut sheet 911 is set within the range of 55.degree. to 85.degree.. The above mentioned published application also teaches a friction member made of an elastic sheet and fixed at its lower end with the same angular condition as described above, instead of being rotatably supported by the shaft 932.
The friction member of such configuration, however, requires a very fine adjustment to successfully separate the cut sheets. According to an experiment conducted by the inventors of the present invention, the rebounding force of the spring 934 toward its initial position should be set weak enough to prevent residual deformation of the cut sheet caused by the friction member 933. On the other hand, the rebounding force should be strong enough to increase the frictional force between the cut sheet 911 and the friction member 933 so as to ensure the separation. It is difficult to satisfy both of these contradictory conditions. Moreover, physical properties of cut sheets such as resiliency or frictional coefficient vary depending on their quality and thickness, causing differences in frictional resistance even with the use of the same friction member.
Also, the friction member 933 in this prior art arrangement is disposed downstream in a feeding direction and away from a nipping position where the feeding roller 931 presses and pushes the cut sheet 911 forward. The cut sheet is, when contacted against the friction member 933, warped and separated from the other cut sheets. Since the friction member 933 is distanced from the feeding roller 931 widely enough to allow the cut sheet 911 to flexibly deform therebetween, the cut sheet 911 can easily escape from the friction member 933 which is being tilted backward. This configuration helps to prevent residual deformation of the cut sheets 911 irrespective of their thickness.
Still, the sheet feeder unit of this prior art is not fully capable of stably separating cut sheets 911 and sometimes supplies the cut sheets in plural one upon another for the following reason. Since the friction member 933 has an even surface against which the cut sheet 911 is contacted, the more the leading end of the cut sheet 911 pushes the friction member 933 and approaches a free end thereof, the more the contacting angle therebetween widens, to let the cut sheet 911 pass, causing the frictional force to become less than a desired degree. Multiple cut sheets 911 may thus slip through the friction member 933 from time to time without being fully separated from each other. Although the setting of the angle between the cut sheet 911 and the friction member 933 helps prevent such slippage to some extent, it is not enough to fully prevent faulty feeding of cut sheets, especially those being less resilient or having a greater frictional coefficient due to their quality or thickness.
The conditions required for successfully feeding sheets may be satisfactorily set, though it is only temporary as the sheet feeding performance is dependent on various ambient conditions. For example, a cut sheet made of paper which has high absorbency may be less smoothly conveyed. An unabsorbent cut sheet made of synthetic resin for an overhead projector may be also affected by humidity on its surface, which may cause the cut sheet to be fed one upon another. Even the fiber directions (whether parallel or right-angled with respect to the sheet feeding direction) of manufactured paper may slightly affect the resiliency of the cut sheet, causing instability in sheet feeding performance.
Further, the friction member 933 of the prior art apparatus has an even friction surface with respect to a whole width of the leading edge of the cut sheet 911 contacted thereto. For this reason, when the conditions are set to provide enough frictional force in order to fully separate cut sheets of small size, cut sheets of greater size cannot be smoothly fed because of too much frictional force. On the other hand, when the conditions are set appropriately for feeding cut sheets of great size, cut sheets of small size may be fed one upon another due to insufficient frictional force.
Further, when a cut sheet having its leading edge curled downwardly is fed, it is more likely that the cut sheet is curled even more when pushed against the friction member 933 because of its even frictional surface in the prior art arrangement, which can leads to a paper jam or cause the cut sheet to be crumpled or stuck to the following sheets.